1. The Field of the Invention
This application relates to optical transmitters and, more particularly, to systems and methods for reducing errors caused by nonlinear effects experienced by signals during transmission through optical fiber.
2. The Relevant Technology
In long haul dense wavelength division multiplex (DWDM) transmission systems, dispersion tolerance and high optical signal to noise ratio (OSNR) are two key parameters. Normally, the two requirements are contradicted. On one hand, the power in a WDM transmission system needs to be controlled within a certain level to reduce nonlinear effects for wider dispersion tolerance and better performance; on the other hand, systems need high power for high optical signal to noise ratio (OSNR). The nonlinear effects include self phase modulation (SPM), cross phase modulation (XPM) and four waves mixing (FWM), stimulated Brillouin scattering (SBS), and stimulated Raman scattering (SRS).
In most long haul DWDM transmission systems, optical signal is intensity modulated. Amplitude of “1” bits and “0” bits is different. Since nonlinear effects relate to optical power, “1” bits and “0” bits have different nonlinear effects. For example, longer “1” patterns have different nonlinear effects compared to short “1” pattern. Nonlinear effects, such as SPM and XPM, give more penalty if dispersion is present. Furthermore, it cannot be compensated by dispersion compensation fiber. Generally speaking, the dispersion tolerance of WDM systems is reduced due to nonlinearities.
Different methods have been proposed to improve dispersion tolerance by reducing nonlinear effects and OSNR performance, such as distributed Raman amplifiers, return-to-zero (RZ) transmission format, and differential phase shift key (DPSK) format. Meanwhile, in all of the above methods, the dispersion map of the fiber should be carefully designed.
In order to achieve wider dispersion tolerance in a WDM system, higher tolerance to nonlinear effects and good OSNR performance are needed. None of the =existing methods can realize the above features simultaneously without requiring complicated system design. For example, DPSK can improve tolerance to nonlinear effects, but it does little to improve dispersion tolerance. A distributed Raman amplifier, can improve dispersion tolerance, but not tolerance to nonlinear effects; For optical duobinary signal, in linear case, the dispersion tolerance can reach +/−3000 ps/nm, but in presence of nonlinear effects its tolerance to dispersion is greatly reduced. Compensation for nonlinear effects using electronic dispersion compensation (EDC) is also difficult. Use of the RZ format can improve a system's OSNR performance, but the dispersion tolerance is reduced.
In view of the foregoing it would be an advancement in the art to provide a simple and cost-effective system that effectively reduces errors due both to dispersion and nonlinear effects.